The present invention relates to a camera provided with an automatic focusing system wherein an objective lens assembly of the camera is automatically shifted in an axial direction thereof to an optimum position for focusing a target object to be photographed on a focal plane of the camera, in accordance with a signal indicative of a distance between the target object and the camera.
Conventionally, there have been proposed various types of the above described camera provided with the automatic focusing system and one of which is disclosed in a Japanese Patent Publication No. 7828/1967. The automatic focusing system disclosed therein includes a movable objective lens which moves between a hyperfocal or infinity position and near position relative to the focal plane, and a distance measuring device operatively coupled to the objective lens to assume a position corresponding to that of the movable objective lens. As the movable objective lens moves from the hyperfocal position to the near position, a focus signal indicative of the focus condition of the objective lens for the target object varies and the objective lens is stopped when the focus signal assumes a value indicative of a proper focus condition.
If a conventional focus detecting device fails to obtain a proper focus condition signal due to the fact that the target object is too dark to be detected or the portion to be focused is a plane wall having no distinctive contrast, or by some other reasons, the movable objective lens will move to the extreme near position, or a shutter of the camera may not be released where such proper focus signal is utilized for allowing the release of shutter mechanism.
From this aspect, this known system provides a provisional distance setting device which, when no proper focus signal is produced from a focus detecting device, produces a provisional distance signal for setting the movable objective lens to a pan-focal position at which the lens is held in position to be focused on the target object located at a middle distance away from the camera, so that substantially in-focus images can be obtained for the object at any distance by virtue of depth of field.
However, according to this publication, the provisional distance setting device operates after the objective lens has been completely moved to the extreme near position. Therefore, when it is necessary to set the objective lens to the pan-focal position due to the failure in operation of the focus detecting device, the objective lens is so designated as to move from the hyperfocal position to the near position and then return back to the pan focal position. Thus, it takes a considerably long time before the camera is ready to release the shutter mechanism. Furthermore, in the case where such provisional distance setting device is to be employed in the automatic focusing system of the type in which the objective lens is driven from either extreme to a proper focus position in accordance with a signal obtained prior to the actuation of the lens, it is necessary to provide means for moving the objective lens, for example, from the near position to the pan-focal position, in addition to means for moving the same, for ordinary control, from the hyperfocal position to the near position. Thus, the mechanism for operating the movable lens will become considerably complicated.
Moreover, in the case where the provisional distance setting device of the above described type is employed in the automatic focusing system operable on the basis of the principle of triangulation and using a rotating mirror for scanning the field, it is necessary to provide means for rotating the mirror from its initial position to a final position to detect whether or not the target object is within the entire range of distance and also means for rescanning the mirror back to a required position, if no target object is detected within said range, for setting the objective lens to the pan-focal position. If the camera is so arranged as to position the objective lens to the pan-focal position during the first scanning of the mirror, it is necessary to provide additional device for detecting the unfavorable condition of the target object, e.g. darkness or low contrast and a circuit for transacting the output signal from such detecting device.